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Published: Jul 17, 2019 License: MIT Imports: 4 Imported by: 3,026


Exponential Backoff GoDoc Build Status Coverage Status

This is a Go port of the exponential backoff algorithm from Google's HTTP Client Library for Java.

Exponential backoff is an algorithm that uses feedback to multiplicatively decrease the rate of some process, in order to gradually find an acceptable rate. The retries exponentially increase and stop increasing when a certain threshold is met.




  • I would like to keep this library as small as possible.
  • Please don't send a PR without opening an issue and discussing it first.
  • If proposed change is not a common use case, I will probably not accept it.



Package backoff implements backoff algorithms for retrying operations.

Use Retry function for retrying operations that may fail. If Retry does not meet your needs, copy/paste the function into your project and modify as you wish.

There is also Ticker type similar to time.Ticker. You can use it if you need to work with channels.

See Examples section below for usage examples.




View Source
const (
	DefaultInitialInterval     = 500 * time.Millisecond
	DefaultRandomizationFactor = 0.5
	DefaultMultiplier          = 1.5
	DefaultMaxInterval         = 60 * time.Second
	DefaultMaxElapsedTime      = 15 * time.Minute

Default values for ExponentialBackOff.

View Source
const Stop time.Duration = -1

Stop indicates that no more retries should be made for use in NextBackOff().


View Source
var SystemClock = systemClock{}

SystemClock implements Clock interface that uses time.Now().


func Retry

func Retry(o Operation, b BackOff) error

Retry the operation o until it does not return error or BackOff stops. o is guaranteed to be run at least once.

If o returns a *PermanentError, the operation is not retried, and the wrapped error is returned.

Retry sleeps the goroutine for the duration returned by BackOff after a failed operation returns.

// An operation that may fail.
operation := func() error {
	return nil // or an error

err := Retry(operation, NewExponentialBackOff())
if err != nil {
	// Handle error.

// Operation is successful.

func RetryNotify

func RetryNotify(operation Operation, b BackOff, notify Notify) error

RetryNotify calls notify function with the error and wait duration for each failed attempt before sleep.


type BackOff

type BackOff interface {
	// NextBackOff returns the duration to wait before retrying the operation,
	// or backoff. Stop to indicate that no more retries should be made.
	// Example usage:
	// 	duration := backoff.NextBackOff();
	// 	if (duration == backoff.Stop) {
	// 		// Do not retry operation.
	// 	} else {
	// 		// Sleep for duration and retry operation.
	// 	}
	NextBackOff() time.Duration

	// Reset to initial state.

BackOff is a backoff policy for retrying an operation.

func WithMaxRetries

func WithMaxRetries(b BackOff, max uint64) BackOff

WithMaxRetries creates a wrapper around another BackOff, which will return Stop if NextBackOff() has been called too many times since the last time Reset() was called

Note: Implementation is not thread-safe.

type BackOffContext added in v1.1.0

type BackOffContext interface {
	Context() context.Context

BackOffContext is a backoff policy that stops retrying after the context is canceled.

func WithContext added in v1.1.0

func WithContext(b BackOff, ctx context.Context) BackOffContext

WithContext returns a BackOffContext with context ctx

ctx must not be nil

type Clock

type Clock interface {
	Now() time.Time

Clock is an interface that returns current time for BackOff.

type ConstantBackOff

type ConstantBackOff struct {
	Interval time.Duration

ConstantBackOff is a backoff policy that always returns the same backoff delay. This is in contrast to an exponential backoff policy, which returns a delay that grows longer as you call NextBackOff() over and over again.

func NewConstantBackOff

func NewConstantBackOff(d time.Duration) *ConstantBackOff

func (*ConstantBackOff) NextBackOff

func (b *ConstantBackOff) NextBackOff() time.Duration

func (*ConstantBackOff) Reset

func (b *ConstantBackOff) Reset()

type ExponentialBackOff

type ExponentialBackOff struct {
	InitialInterval     time.Duration
	RandomizationFactor float64
	Multiplier          float64
	MaxInterval         time.Duration
	// After MaxElapsedTime the ExponentialBackOff stops.
	// It never stops if MaxElapsedTime == 0.
	MaxElapsedTime time.Duration
	Clock          Clock
	// contains filtered or unexported fields

ExponentialBackOff is a backoff implementation that increases the backoff period for each retry attempt using a randomization function that grows exponentially.

NextBackOff() is calculated using the following formula:

randomized interval =
    RetryInterval * (random value in range [1 - RandomizationFactor, 1 + RandomizationFactor])

In other words NextBackOff() will range between the randomization factor percentage below and above the retry interval.

For example, given the following parameters:

RetryInterval = 2
RandomizationFactor = 0.5
Multiplier = 2

the actual backoff period used in the next retry attempt will range between 1 and 3 seconds, multiplied by the exponential, that is, between 2 and 6 seconds.

Note: MaxInterval caps the RetryInterval and not the randomized interval.

If the time elapsed since an ExponentialBackOff instance is created goes past the MaxElapsedTime, then the method NextBackOff() starts returning backoff.Stop.

The elapsed time can be reset by calling Reset().

Example: Given the following default arguments, for 10 tries the sequence will be, and assuming we go over the MaxElapsedTime on the 10th try:

Request #  RetryInterval (seconds)  Randomized Interval (seconds)

 1          0.5                     [0.25,   0.75]
 2          0.75                    [0.375,  1.125]
 3          1.125                   [0.562,  1.687]
 4          1.687                   [0.8435, 2.53]
 5          2.53                    [1.265,  3.795]
 6          3.795                   [1.897,  5.692]
 7          5.692                   [2.846,  8.538]
 8          8.538                   [4.269, 12.807]
 9         12.807                   [6.403, 19.210]
10         19.210                   backoff.Stop

Note: Implementation is not thread-safe.

func NewExponentialBackOff

func NewExponentialBackOff() *ExponentialBackOff

NewExponentialBackOff creates an instance of ExponentialBackOff using default values.

func (*ExponentialBackOff) GetElapsedTime

func (b *ExponentialBackOff) GetElapsedTime() time.Duration

GetElapsedTime returns the elapsed time since an ExponentialBackOff instance is created and is reset when Reset() is called.

The elapsed time is computed using time.Now().UnixNano(). It is safe to call even while the backoff policy is used by a running ticker.

func (*ExponentialBackOff) NextBackOff

func (b *ExponentialBackOff) NextBackOff() time.Duration

NextBackOff calculates the next backoff interval using the formula:

Randomized interval = RetryInterval +/- (RandomizationFactor * RetryInterval)

func (*ExponentialBackOff) Reset

func (b *ExponentialBackOff) Reset()

Reset the interval back to the initial retry interval and restarts the timer.

type Notify

type Notify func(error, time.Duration)

Notify is a notify-on-error function. It receives an operation error and backoff delay if the operation failed (with an error).

NOTE that if the backoff policy stated to stop retrying, the notify function isn't called.

type Operation

type Operation func() error

An Operation is executing by Retry() or RetryNotify(). The operation will be retried using a backoff policy if it returns an error.

type PermanentError added in v1.1.0

type PermanentError struct {
	Err error

PermanentError signals that the operation should not be retried.

func Permanent added in v1.1.0

func Permanent(err error) *PermanentError

Permanent wraps the given err in a *PermanentError.

func (*PermanentError) Error added in v1.1.0

func (e *PermanentError) Error() string

type StopBackOff

type StopBackOff struct{}

StopBackOff is a fixed backoff policy that always returns backoff.Stop for NextBackOff(), meaning that the operation should never be retried.

func (*StopBackOff) NextBackOff

func (b *StopBackOff) NextBackOff() time.Duration

func (*StopBackOff) Reset

func (b *StopBackOff) Reset()

type Ticker

type Ticker struct {
	C <-chan time.Time
	// contains filtered or unexported fields

Ticker holds a channel that delivers `ticks' of a clock at times reported by a BackOff.

Ticks will continue to arrive when the previous operation is still running, so operations that take a while to fail could run in quick succession.

// An operation that may fail.
operation := func() error {
	return nil // or an error

ticker := NewTicker(NewExponentialBackOff())

var err error

// Ticks will continue to arrive when the previous operation is still running,
// so operations that take a while to fail could run in quick succession.
for _ = range ticker.C {
	if err = operation(); err != nil {
		log.Println(err, "will retry...")


if err != nil {
	// Operation has failed.

// Operation is successful.

func NewTicker

func NewTicker(b BackOff) *Ticker

NewTicker returns a new Ticker containing a channel that will send the time at times specified by the BackOff argument. Ticker is guaranteed to tick at least once. The channel is closed when Stop method is called or BackOff stops. It is not safe to manipulate the provided backoff policy (notably calling NextBackOff or Reset) while the ticker is running.

func (*Ticker) Stop

func (t *Ticker) Stop()

Stop turns off a ticker. After Stop, no more ticks will be sent.

type ZeroBackOff

type ZeroBackOff struct{}

ZeroBackOff is a fixed backoff policy whose backoff time is always zero, meaning that the operation is retried immediately without waiting, indefinitely.

func (*ZeroBackOff) NextBackOff

func (b *ZeroBackOff) NextBackOff() time.Duration

func (*ZeroBackOff) Reset

func (b *ZeroBackOff) Reset()

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